Conventional arrangements for cooling superconducting magnets include a cryogen vessel partly filled with a bath of liquid cryogen such as helium. Windings of the superconducting magnet are partially immersed in the liquid cryogen, to hold them at a temperature of approximately the boiling point of the cryogen.
Such arrangements provide a quasi-isothermal environment within the cryogen vessel. Ambient heat is prevented from reaching the windings by a cryostat comprising the cryogen vessel, an outer vacuum chamber (OVC) enclosing the cryogen vessel and mechanical supports arranged to retain the magnet, the cryogen vessel and the OVC in required relative positions. One or more thermal radiation shields are typically provided in the space between the cryogen vessel and the OVC.
The vacuum within the OVC minimizes convection losses, and thermal conduction through the mechanical supports is minimized by appropriate material choice and dimensions. Such thermal conduction as remains may be intercepted by an active cooler such as a cryogenic refrigerator thermally linked to the shield(s) and/or the cryogen vessel.
Through careful design, the thermal influx reaching the “cold mass”, that is, the liquid cryogen and everything in contact with it, may be reduced to less than 1 W. Even so, care must be taken to ensure that this thermal influx does not reach the superconducting windings. Partial immersion in liquid cryogen, such as helium, together with convection cooling and recondensation of cryogen vapor by a cryogenic refrigerator conventionally ensures that the thermal influx does not reach the superconducting windings.
An outer surface of the cryogen vessel may be coated in a low emissivity coating, such as aluminum foil, which will reflect thermal radiation reaching the cryogen vessel, contributing to keeping the thermal influx away from the superconducting windings.
Conventional design philosophy has been to minimize the surface area of cryogen vessels, and to insulate them from incident radiant heat, including by applying highly reflective, low thermal emissivity surface coatings to reflect incident thermal radiation.
As used herein, the term “thermal radiation” and similar terms are used to refer to electromagnetic radiation in the thermal infra-red range of wavelengths, approximately 8 to 14 micrometers. The terms “thermal emissivity” and similar terms are used to refer to emissivity of thermal radiation.
More recently, superconducting magnet cooling arrangements have been devised which do not require a cryogen vessel containing a bath of liquid cryogen. Local coil cooling solutions are provided instead. For example, pipe cooled systems, sometimes referred to as cooling loops, may involve relatively small quantities of liquid cryogen such as helium circulating from a small cryogen reservoir through pipe and manifold systems in thermal contact with the superconducting windings.
FIG. 1 shows a cross-section through a conventional superconducting magnet cooling arrangement employing a local coil cooling solution. A hollow cylindrical OVC 10 is provided, housing superconducting magnet windings 20. A thermal radiation shield 12 is provided within the OVC, and solid-state thermal insulation 14, such as multi-layer aluminized polyester sheet known as Superinsulation® may be provided between the OVC inner surface and the outer surface of the thermal radiation shield.
The local coil cooling solution typically, and as illustrated, comprises a cryogen vessel 22 provided with access turret 24 and an external refrigerator 26, thermally linked by thermal bus 28 to a recondenser (not visible) exposed to the interior of the cryogen vessel 22. In alternative arrangements, a refrigerator may be connected directly to the cryogen vessel. The cryogen vessel 22 provides cooled, preferably liquid, cryogen through a tube 30 to a manifold 32. The manifold distributes the cooled cryogen to cooling loops arranged in thermal contact with superconducting windings of the magnet, which cooling loops operate according to the conventional and well-documented thermal convection method.
Attempts may be made to further shield the magnet windings 20 from radiant heat emitted by the inner surface of the thermal radiation shield 12, or other surfaces. For example, the magnet windings may be wrapped in a tape of low-emissivity material. However, such attempts have not been found to be completely successful. A certain amount of heat has always been found to make its way between turns of tape, or otherwise, to the structure shielded by the tape.
All heat reaching the interior of the thermal radiation shield(s) may impinge upon, and be absorbed by, the windings themselves. A small cryogen reservoir used in such a system may itself be coated in a low thermal emissivity coating, but this will not reduce the incidence of thermal radiation onto the superconducting windings, unlike such a coating applied to the cryogen vessel of conventional cryogen-bath arrangements.